1. Field of the Invention
The present invention relates to a non-zero dispersion shifted fiber (hereinafter, abbreviated to xe2x80x9cNZ-DSFxe2x80x9d) used for wavelength division multiplexing (hereinafter, abbreviated to xe2x80x9cWDMxe2x80x9d), and in particular, relates to an NZ-DSF showing chromatic dispersion characteristics enabling transmission over an S-band (short wavelength band, 1460 to 1530 nm) to C-band (conventional band, 1530 to 1565 nm) to L-band (long wavelength band, 1565 to 1625 nm), and showing a relative dispersion slope (hereinafter, abbreviated to xe2x80x9cRDSxe2x80x9d) which is almost the same RDS of a slope compensating dispersion compensation fiber (hereinafter, abbreviated to xe2x80x9cSC-DCFxe2x80x9d) conventionally used for a single mode optical fiber.
2. Description of Related Art
Capacities of optical transmission systems have been increasing significantly using the WDM method. In the WDM method, reduction of non-linear effects and control of chromatic dispersion are required in transmission optical fibers.
In general, the non-linear effects in an optical fiber is represented by n2/Aeff, where n2 is a non-linear refractive index of the optical fiber and Aeff is an effective core area of the optical fiber. Therefore, the non-linear effects are inversely proportional to Aeff. Accordingly, various optical fibers are developed such as optical fibers having enlarged effective core area Aeff, optical fibers having reduced dispersion slope, and optical fibers which compensates dispersion slopes.
In order to increase the transmission capacity based on the WDM method, two methods are mainly used. The first method is a method of increasing the number of waves for multiplexing, and the second method is a method of improving the transmission speed.
As the method of increasing the number of waves for multiplexing, there is a trend of broadening the wavelength band for transmission. A 1550 nm-band is mainly used as a wavelength band for the WDM method. In the 1550 nm-band, a band known as the C-band has been widely used, but in recent years, there has been a trend of the use of the L-band and S-band for communication.
Therefore, various optical fibers are proposed such as optical fibers for use in C-band and L-band, and optical fibers having larger chromatic dispersion for use in S-, C- and L-bands.
Furthermore, in order to increase transmission speed, the transmission system is shifted from 2.5 Gb/s to 10 Gb/s, and also to 20 Gb/s or 40 Gb/s of the high-speed transmission system.
Several optical fibers for the transmission over S- to C- to L-bands have been already proposed. Examples of chromatic dispersion characteristics of such optical fibers are shown in FIG. 12.
One is a system of an SC-DCF in combination to a single-mode fiber for use in the 1.3 xcexcm band (hereinafter abbreviated to xe2x80x9c1.3 SMFxe2x80x9d). Using the SC-DCF having the RDS wavelength characteristics which is almost the same RDS wavelength characteristics of 1.3 SMF, dispersion compensation can be over wide range. The RDS is a parameter defined by the following expression 1.                     RDS        =                                            Dispersion              ⁢                              xe2x80x83                            ⁢              slope                                      Chromatic              ⁢                              xe2x80x83                            ⁢              dispersion                                ⁡                      [                          nm                              -                1                                      ]                                              (        1        )            
When the SC-DCF having the same RDS as the 1.3 SMF and having a chromatic dispersion value which is positive or negative so as to be opposite to that of the 1.3 SMF, is used in the system, dispersion slope in addition to chromatic dispersion can be compensated.
However, the chromatic dispersion at the wavelength 1550 nm band of 1.3 SMF is very a large value of +17 ps/nm/km. According to increasing the transmission speed, the interval of dispersion compensation is required to be shorter. When the 1.3 SMF has 2.5 Gb/s of transmission speed, the transmission distance can be about 1000 km. However, when the 1.3 SMF has 10 Gb/s of transmission speed, the transmission distance is 50 km, and when the 1.3 SMF has 40 Gb/s, the transmission distance is 4 km.
In view of limiting the transmission distance by accumulated dispersion, the NZ-DSF having characteristics shown in continuous line (a) and dashed line (b) of FIG. 14 is superior to the 1.3 SMF. However, a conventional NZ-DSF has a zero dispersion wavelength around 1500 nm, and as a result, the WDM transmission cannot be carried out at the S-band. To solve this problem, recently, an optical fiber, taking the S-band transmission into consideration, was developed.
For example, in an optical fiber (trade name: xe2x80x9cTeralight(trademark)xe2x80x9d trade mark) having characteristics shown in chain line (c) of FIG. 14, the chromatic dispersion at the wavelength 1550 nm band is set to about +8 ps/nm/km, resulting in the S-band transmission. However, the optical fiber has larger chromatic dispersion than the conventional NZ-DSF over S-band to C-band to L-band. As a result, the optical fiber has a shorter transmissible distance without dispersion compensation than the conventional NZ-DSF.
Furthermore, as an optical fiber in which the WDM transmission can be carried out to the S-band range, an NZ-DSF in which a dispersion slope is decreased up to 0.02 ps/nm/km is reported. The chromatic dispersion characteristics are shown in a chain double-dashed line of FIG. 14. The optical fiber shows the chromatic dispersion which is less than that of the conventional NZ-DSF at the L-band, and can most flexibly be used for wide band and high-speed transmission.
However, even if the above type of optical fiber is used, when high-speed transmission of 40 Gb/s is carried out, an SC-DCF is necessary to dispersion-compensate. The RDS of the optical fiber is 0.036 to 0.040 nmxe2x88x921, it is necessary to design an SC-DCF only for this optical fiber. Using the SC-DCF causes increased manufacturing cost all over optical fiber transmission path.
The present invention is provided in view of the problems described above, and an object is the provision of an non-zero dispersion shifted fiber having a chromatic dispersion enabling transmission over S-band to C-band to L-band, and having almost the same RDS as a normal single-mode optical fiber over C-band to L-band, in order to provide an optical transmission path in which high-speed transmission can be carried out without an SC-DCF only for the non-zero dispersion shifted fiber.
To achieve the above object, the first aspect of the present invention is an optical fiber having a chromatic dispersion of +1.0 ps/nm/km or more at 1460 nm wavelength band, a dispersion slope of 0.04 ps/nm2/km or less at 1550 nm wavelength band, and a cutoff wavelength of 1450 nm or less, wherein a relation of an RDS, which is a value of the dispersion slope to the chromatic dispersion, to a wavelength xcex is xe2x88x921.67xc3x9710xe2x88x925xcex+0.0300xe2x89xa7RDS(xcex) xe2x89xa7xe2x88x921.67xc3x9710xe2x88x925xcex+0.0285.
According to the above aspect, the obtained optical fiber has the chromatic dispersion characteristics enabling optical transmission over S-band to C-band to L-band, resulting in a wavelength multiplexing transmission, and the obtained optical fiber has almost the same RDS as a normal single-mode optical fiber and the SC-DCF thereof over C-band to L-band, as a result, the chromatic dispersion and the dispersion slope can be compensated using the SC-DCF for normal single-mode optical fiber over C-band to L-band.
The second aspect of the present invention is an optical fiber having a chromatic dispersion of +1.0 ps/nm/km or more at the 1460 nm wavelength band, a dispersion slope of 0.04 ps/nm2/km or less at 1550 nm wavelength band, and a cutoff wavelength of 1450 nm or less, comprising wavelength bandwidth having wavelength bandwidths containing over 115% and less than 115% of dispersion slope compensating coefficient, or having wavelength bandwidths containing over 100% and less than 100% of dispersion slope compensating coefficient in a wavelength bandwidth in which dispersion compensation is carried out; a dispersion slope compensating coefficient at long-wavelength side being 80% to 150% or being 100% to 130% in a wavelength bandwidth in which dispersion compensation is carried out; and a dispersion slope-compensating coefficient at short-wavelength side being 170% or less or 150% or less in a wavelength bandwidth in which dispersion compensation is carried out using a dispersion compensating optical fiber.
According to the above aspect, in the optical fiber, the wavelength multiplexing transmission over S-band to C-band to L-band can be carried out since the optical fiber has the chromatic dispersion characteristics in which optical transmission can be carried out over S-band to C-band to L-band, and further, since the optical fiber over C-band to L-band has the same RDS as a normal single-mode optical fiber and an SC-DCF thereof have, the chromatic dispersion and the dispersion slope can be compensated using the SC-DCF for normal single-mode optical fiber.
In the optical fiber according to the second aspect, the dispersion compensating optical fiber may be for dispersion-compensating a single-mode optical fiber such as one for use at 1.3 xcexcm.
In the optical fiber according to the first or second aspect, an effective core area may be 35 to 60 xcexcm2, or a mode field diameter is 7 to 9 xcexcm.
The optical fiber according to the first or second aspect comprises a central core, two or more ring cores provided on the circumference of the center core, and a cladding provided on the circumference of the outermost ring core, wherein two or more ring cores have different refractive indices, and when a refractive index of the central core is designated as n1, refractive indices of the ring cores are designated as n2, n3, . . . , from the central core side to the outside, and a refractive index of the cladding is designated as nc, a relation thereof may be n1 greater than n3 greater than nc greater than n2.
In the above optical fiber, a relative refractive index difference of the central core may be 0.4 to 0.6%.
In the above optical fiber, when xcex94n(r) indicates a relative refractive index difference (%) of the n-th core, r indicates a radius (xcexcm) of the optical fiber, rn indicates a radius (xcexcm) of the n-th core, and rnxe2x88x921 indicates a radius (xcexcm) of the (nxe2x88x921)-th core, in a refractive index volume Vn defined by expression (2), a ratio of a refractive index volume in the ring core area V2 which is adjacent to the central core to a refractive index volume in the central core area V1 (V2/V1) may be xe2x88x923.0 to xe2x88x921.0.                               V          n                =                              ∫                          r                              n                -                1                                                    r              n                                ⁢                                                                      Δ                  n                                ⁡                                  (                  r                  )                                            ·              r                        ⁢                          ⅆ              r                                                          (        2        )            
In the optical fiber having a relative refractive index difference of the central core of 0.4 to 0.6%, a refractive index volume in the ring core area V2 which is adjacent to the central core to a refractive index volume in the central core area V1 (V2/V1) may be xe2x88x922.0 to xe2x88x921.0.
Furthermore, an optical transmission path is formed by combining any one of the above optical fibers and a dispersion compensating optical fiber.
According to the optical transmission path, high-speed transmission having 40 Gb/s over C-band to L-band can be carried out.
In the above optical fiber, more preferably, there are wavelength bandwidth of more than 100% compensating coefficient of dispersion slope and wavelength bandwidth of less than 100% compensating coefficient of dispersion slope; and compensating coefficient of dispersion slope is 100 to 130% at the long wavelength side of wavelength bandwidth to be subjected to dispersion compensation, and compensating coefficient of dispersion slope is 150% or less at the short wavelength side of wavelength bandwidth to be subjected to dispersion compensation. Accordingly, wavelength division multiplexing transmission can be carried out over S-band to C-band to L-band, and the optical fiber can compensate chromatic dispersion and dispersion slope over C-band to L-band using the SC-DCF for the normal single-mode optical fiber. Moreover, since an optical transmission path is formed by combining any one of the above optical fibers and a dispersion compensating optical fiber, high-speed transmission having 40 Gb/s over C-band to L-band can be carried out.